The aims of the present paper are the following: firstly, to describe the psychological treatment administered to sexual offenders in Spain; secondly, to assess the effectiveness of the application of this psychological treatment in the prison of Brians (Barcelona). For this purpose, two equivalent groups were selected: a treatment group of 49 subjects who received the whole treatment program, and an untreated control group of 74 subjects. These groups were selected taking into account various risk factors in order to guarantee group comparability. The main results not only show the efficacy of the cognitive-behavioural program for sexual offenders but also that the effectiveness of this program exceeds the average of similar programs in the meta-analytic reviews performed to date.